Replace my courses login form with standard Wordpress login form

[yscik]

Fixes #3312

Changes proposed in this Pull Request

• Use wp_login_form to render login form in My Courses page

TODO

• Handle wrong password redirect (Don't redirect to wp-login after wrong password entry #407)
• Display error messages on failed attempt

Testing instructions

• Visit the My Courses page and use the login form

Removed Hooks

These hooks are removed in favor of their core counterparts:

• sensei_login_form_inside_before -> login_form_top
• sensei_login_form_inside_after_password_field -> login_form_middle
• sensei_login_form_inside_after -> login_form_bottom

We could also just deprecate them, and set it up so they are still called from these core hooks, but that might do more harm if the user added callbacks to both the sensei login and core login hooks.

Screenshot / Video